Generation of light in solid-state lasers is based on electronic transitions in active ions doped into a host material. Depending on the type of active ion, and on the host material, different wavelengths are achievable from the transition.
The wavelengths obtained from solid-state lasers are typically in the near infrared region of the electromagnetic spectrum. In order to reach shorter wavelength, for example visible radiation, various techniques based on non-linear interactions are widely employed.
One commonly used class of active ions are rare earth metals. These metal ions are doped into a host material, which is either crystalline or amorphous, to obtain an active laser medium. Examples of rare earth metals used in solid-state lasers are Nd, Yb, Ho, Er, Tm and Pr. However, other rare earths are also used in some cases.
Due to the environment of the crystal lattice or the amorphous host, each of the electronic states of the active ions is splitted into a manifold. Both the excited states of the active ions and the ground state are in fact manifolds.
Normally, when a transition to the ground state is used in a laser, the laser is called a three level laser. The reason is that three levels are involved in the process of generation of light. The first level involved is the pump level, to which the ions are excited by means of an optical pump source. The ions are rapidly transferred from this pump level to a lower energy level, which is called the upper laser level. The lifetime of this level is comparatively long compared to the other levels involved. Therefore, this upper laser level is often referred to as a “metastable” level. Transition from the upper laser level to a lower energy level may now be stimulated by light at the appropriate frequency (of the appropriate energy). The transition from the upper laser level takes place to the lower laser level, which in the case of three level lasers is the ground state of the active ion.
When the lower laser level is an excited state of the active ion, the laser is called a four level laser, since four levels are involved in the generation of light. In case of a four level laser, the lower laser level is quickly emptied by spontaneous transition of the ions to the ground state. Consequently, four level laser are typically more efficient than three level lasers, because population inversion (the requisite for laser action) is more easily obtained.
One class of three level lasers is the so-called quasi three level lasers. In this case, the lower laser level is, in fact, within the manifold of the ground state. However, the lower laser level is slightly above the actual zero level. Consequently, the lower laser level has some thermal population at equilibrium. This thermal population causes reabsorption of light at the laser wavelength, whence the efficiency is lower than for four level lasers. However, the efficiency is still higher than for “true” three level lasers, where the lower laser level is the zero level.
The 4F3/2–4I11/2 (as well as 4F3/2–4I13/2) transition in Nd doped laser crystals or glasses is commonly utilised in highly efficient four level solid state lasers emitting light of wavelengths between 1040 and 1080 nm (1300–1450 nm). This system is a four level system, because the 4I11/2 state is very short-lived and the ions in this level are almost immediately transferred to the ground state.
When using the ground state manifold 4I9/2 of Nd ions as the lower laser level, the laser wavelength decreases to a substantially shorter emission wavelength between 900 and 950 nm. However, a drawback of this approach is the above-mentioned quasi three level nature of the system. It suffers from low gain and temperature-dependent reabsorption losses due to the thermal population of the lower laser level. Another quasi-three level laser system that has achieved attention is the Yb doped laser crystal or glass (laser transition 2F5/2–2I7/2). This system has some advantages, such as low pump defect (small energy difference between the pump level and the upper laser level) and the lack of excited state absorption. The wavelength range that can be covered with Yb-doped materials ranges from 980 to 1070 nm. As mentioned above, there are other rare earth ions that can be used as active ions in solid state lasers. For example Ho or Tm gives a quasi three level laser, but with substantially longer output wavelength (around 2000 nm).
Although quasi three level lasers have lower gain, and hence are typically less efficient and quite sensitive to losses, their overall performance can compete with that of four level lasers, when the laser design is carefully optimised. Optimising includes achieving an optimum balance between high pump absorption and low reabsorption loss, and finding the optimum output coupling. Furthermore, as bright a pump source as possible should be used (preferably laser diodes),
Second harmonic generation is a widely used technique to convert the above infrared lasers into the visible spectrum. Frequency doubling of the output from a four level laser typically leads to the green or red spectral region, while doubling of the output from a three level laser results in blue or green light. Both types of lasers have been implemented in various configurations and some are already commercially available. However, a number of applications need laser light in-between the green and blue spectral regions (480–510 nm) as well as in-between the orange and the green spectral regions (535–600 nm).
Currently, only ion-gas lasers cover the green to blue spectral region, but these systems are bulky with very high power consumption and short lifetime. One such laser is the well known Argon-ion laser. Just recently, a second approach has been proposed, which is frequency doubling laser diodes at around 975 nm. However, diodes with single transversal mode are required in order to obtain sufficiently good performance, and there are serious restrictions for fabrication of such diodes with high enough output power, e.g. the lifetime suffers. Additionally, the output spectrum gets very broad, which is disadvantageous for the non-linear doubling crystals with their narrow wavelength acceptance bandwidth.
Regarding the green to red spectral region, there has been tried a variety of other approaches. One way is to employ dye-lasers. However, the use of dye-lasers is associated with use of various liquid dyes, which are awkward to work with and may even be poisonous. Furthermore, up-conversion lasers or intracavity doubling of Raman shifted solid state lasers have been proposed. Unfortunately, up-conversion lasers suffer from instabilities, and frequency doubling of Raman-lasers require Q-switching to be sufficiently efficient.
Thus, in order to reach the above-mentioned spectral regions, and to achieve a sufficiently high output power, new coherent light sources are needed.